1. Field of the Invention
The present invention relates to a multiple input multiple output (MIMO) or multiple input single output (MISO) wireless communication system (hereinafter referred to as a “MIMO/MISO wireless communication system”), and more particularly to a method and an apparatus for beam forming signals by means of a beam corresponding to an optimum beam, which maximizes performance of a transmitter, to transmit and/or receive the beam-formed signals in an MIMO/MISO wireless communication system employing a beam forming scheme and a space time block coding (STBC) scheme.
2. Description of the Related Art
Next generation wireless communication systems have evolved from voice only communication systems into packet service communication systems that transmits burst packet data to a plurality of user terminals (UTs). Packet service communication systems have been designed to be adapted for transmission of mass data. Moreover, packet service communication systems are being further developed into high-speed packet services.
However, in order to provide a high-speed packet service, a peak throughput as well as an average throughput must be optimized for a smooth transmission of packet data as well as circuit data such as voice service data.
In providing a high-speed packet service, it is also important to increase a data rate and to enhance transmission reliability. In this regard, a multiple antenna scheme is as a means to increase the data rate and to enhance the transmission reliability. The multiple antenna scheme is capable of overcoming limitations on bandwidth resources in a frequency domain by utilizing a space domain.
In addition, a smart antenna schemes in which signals are received correspondingly to a predetermined direction of arrival (DOA) to perform beam forming of the received signals when there is a correlation between receive antennas. Smart antenna schemes for reception are known as receive beam forming schemes and are suitable to receive uplink signals, that is, signals transmitted from a UT to a base station (hereinafter referred to as BS), in the BS rather than the UT.
In other words, although it is difficult to provide a UT with a plurality of receive antennas due to limitations such as hardware minimization and/or manufacturing costs, the BS is can be easily equipped with a plurality of receive antennas without having to worry about hardware minimization and/or manufacturing costs so it is preferable to apply the smart antenna scheme to the BS.
Several schemes exist for minimizing error. For example, a minimum mean square error MMSE scheme is optimal for maximizing a signal-to-interference-and-noise ratio, (SINR and for maximizing a signal to noise ratio (SNR), a maximum ratio combining (MRC) scheme is optimal.
When signals are transmitted using a plurality of transmit antennas on a transmitter side, that is, downlink signals are transmitted from the BS to the UTs, a transmit beam forming scheme can be used to enhance transmission reliability. In a case of using a transmit beam forming scheme based on the MRC scheme, the SNR is optimized and the same effect as a pre-distortion filter generated by principal eigen vectors of a channel is acquired.
It is possible for the transmit beam forming scheme to provide diversity and array gains and to maximize the transmission reliability on the assumption that exact channel estimation is performed on a receiver side and there is no error in signals fed back from the receiver side to the transmitter side. However, although satisfying the assumption that the exact channel estimation is performed on the receiver side, the transmit beam forming scheme having to perform a complicated eigen-decomposition on the transmitter or receiver side and also suffers because channel information must be fed back from the receiver to the transmitter.
Channel reciprocity, an approach used for preventing feedback of channel status information in a time division duplex (TDD) communication system, is applied in a case using an eigen-beam forming scheme and a directional-beam forming scheme. That is, in using the eigen-beam forming and directional-beam forming schemes, complexity caused by the feedback of channel status information can be minimized because it is possible to analogize, using the channel status of only one link (downlink or uplink), the channel status of the other link (uplink or downlink) when it can be supposed that the downlink channel status is the same as the uplink channel status, in other words, when the downlink and the uplink channels have channel reciprocity, as in the TDD communication system.
However, even in the TDD communication system, the channel reciprocity collapses due to different transmission and reception device characteristics from each other, so efficient calibration for considering the channel statuses is necessary.
Meanwhile, the STBC scheme is a transmission scheme considering a rich scattering channel environment, and supposes that two transmit antennas are used on a transmitter side. Since such an STBC scheme is already well known in the art, a detailed description thereof will be omitted herein (see S. M. Alamouti, “A Simple Transmit Diversity Technique For Wireless Communications,” IEEE Journal of Selected Areas in Communications, Vol. 16, pp. 1451-1458, October 1988).
The STBC scheme can be comparatively simply applied to a transmitter side and easily realize a receiver structure. On this account, it has been already adopted as an option standard in a 3GPP (3rd Generation Partnership Project) standard. However, the STBC scheme suffers because its performance is limited because a correlation between transmit antennas.
In addition, in order to compensate for imperfect channel prediction and imperfect information feedback to a transmitter side, an STBC-beam forming scheme combining the STBC and beam forming schemes with each other has been proposed. For example, see G. Jongren and M. Skoglund, “Combining Beamforming and Orthogonal Space-Time Block Coding,” IEEE Trans. Information Theory, Vol. 48, No. 3, pp. 611-627, March 2000. Hereinafter, the STBC-beam forming scheme will be described with reference to FIG. 1.
FIG. 1 is a block diagram illustrating a structure of an MIMO wireless communication system employing a common STBC-beam forming scheme.
Referring to FIG. 1, the MIMO wireless communication system includes a transmitter 100 and a receiver 150. The transmitter 100 includes a space-time block encoder 111, a beam former 113, a decoder 115 and a plurality of transmit antennas (Tx.ANT) 117-1, . . . , 117-M. The receiver 150 includes a plurality of receive antennas (Rx.ANT) 151-1, . . . , 151-N, a receive processor 153 and an encoder 155.
First, if data to be transmitted from the transmitter 100 to the receiver 150 occurs, the data is transferred to the space-time block encoder 111, and the space-time block encoder 111 encodes the data using an STBC scheme and then outputs the encoded data to the beam former 113. The beam former 113 inputs the signals outputted from the space-time block encoder 111 to perform beam forming of the signals by means of a beam corresponding to beam information outputted from the decoder 115 and then transmits the beam-formed signals to the receiver 150 through the transmit antennas 117-1, . . . , 117-M. The decoder 115 decodes beam information fed back from the receiver 150 and then outputs the decoded beam information to the beam former 113. The fed back beam information is quantization information and will be described later in detail and for the sake of clarity will not described in detail at this point. Moreover, it is noted that a signal reception path from the receiver 150 to the decoder 115 is not shown separately in the drawing.
Noises such as additive white Gaussian noise (AWGN) are added to the signals transmitted from the transmitter 100 while the signals pass through a multipath channel and then the signals with the noises added thereto are received by the receiver 150 through the receive antennas 151-1, . . . , 151-N. The signals received through the receive antennas 151-1, . . . , 151-N are transferred to the receive processor 153 which performs processing of the received signals to output channel information resulted from the processing of the received signals to the encoder 155.
The encoder 155 quantizes the channel information outputted from the receive processor 153 in a predetermined quantization scheme and then transmits the quantized channel information to the transmitter 100. It is also noted that a signal transmission path from the encoder 155 to the transmitter 100 is not shown separately in the drawing.
Research is being undertaken to improve the performance of the STBC-beam forming scheme by individually improving the STBC scheme and the beam forming scheme. As a result of this, an optimum pre-encoder for the space-time block encoder has been proposed, and such an optimum pre-encoder has proved to be implemented by an eigen-beam former for an orthogonal space-time block encoder. For example, see H. Sampath and A. Paulraj, “Linear Precoding for Space-Time Coded Systems with Known Fading Correlations,” IEEE Communications Letters, Vol. 6, No. 6, pp. 239-241, June 2002.
Although the research has been undertaken to improve the STBC-beam forming scheme, the receiver must still feedback the channel information to the transmitter and eigen-analysis for the channel information is necessary.
Hereinafter, a description will be given for a scheme combining a beam scanning scheme and the STBC-beam forming scheme with each other with reference to FIG. 2.
FIG. 2 is a block diagram illustrating a structure of a transmitter in an MIMO wireless communication system employing a combined scheme of common beam scanning and STBC-beam forming schemes.
Referring to FIG. 2, the transmitter in the MIMO wireless communication system includes a modulator 211, a space-time block encoder 213, a serial-to-parallel converter 215, a beam former 217 and a plurality of transmit antennas 219-1, . . . , 219-M.
First, if it is necessary to transmit data from the transmitter to a receiver, the data is transferred to the modulator 21 1which modulates the data using a predetermined modulation scheme and then outputs modulated data signals to the space-time block encoder 213. The space-time block encoder 213 encodes the signals outputted from the modulator 211 using an STBC scheme and then outputs the encoded signals to the serial-to-parallel converter 215. The serial-to-parallel converter 215 parallel-converts the signals outputted from the space-time block encoder 213 and then outputs the converted signals to the beam former 217. The beam former 217 inputs the signals outputted from the serial-to-parallel converter 215, performs beam forming of the signals by means of a beam corresponding to predetermined beam information and then transmits the beam-formed signals to the receiver through the transmit antennas 219-1, . . . , 219-M. Here, the beam information is information to which the beam scanning scheme is applied, that is, to which a channel spatial gain pattern (CSGP) of signals received on the transmitter side is considered, and is generated in consideration of beams having energy above a predetermined threshold value.
However, even though the beam scanning scheme is used together with the STBC-beam forming scheme, the STBC-beam forming scheme still suffers because beams related to unnecessary interference signals must be excluded from the beams having energy above the predetermined threshold value, and satisfactory performance is ensured only when impractical assumptions such as the channel reciprocity are satisfied.